The present invention relates to temperature compensation for use in connection with thermal sensors, such as air flow meters in emissions control.
The use of bi-directional air flow meters in engines is well known and often required for control of emissions. These air flow meters measure the mass of the air inducted into the engine using a flow sensor. A known flow sensor can include a thermally isolated Wheatstone bridge, such as that shown in FIG. 2, which is balanced under a no air-flow condition. When airflow exists, resistors placed downstream of the flow receive more heat from their respective heaters than the resistors placed upstream of the flow. Each resistor has a certain temperature coefficient at which its resistance changes. Thus, with the downstream resistors receiving more heat, their respective resistance will change sooner than the upstream resistors and the Wheatstone bridge then becomes unbalanced. The magnitude of the unbalanced voltage is proportional to the airflow through the meter system.
While the use of resistors provides all inexpensive and easily accessible method of measuring airflow, such resistors, or thermal devices, have an inherent thermal drift. This thermal drift can cause the measurement device to exhibit certain non-linear characteristics. Such iron-linear characteristics call include multiple breakpoints where the drift characteristics change polarity, or exhibiting a positive slope characteristic up to one temperature, and then a negative slope thereafter, or vice versa. Certain of these non-linear characteristics are depicted in the exemplary graphs shown in FIGS. 1A-D. With these non-linear characteristics, the calibration and compensation of the thermal devices or sensors becomes very difficult.
One prior approach to linearize these non-linear characteristics of the thermal devices, or resistors, has been to provide a circuit with two slopes in the compensated voltage characteristics. One system of this type is described in U.S. Pat. No. 5,619,122 which issued on Apr. 8, 1997, to the assignee of the present invention. Also included in this prior approach has been a method of obtaining a change in slope which is dependent on temperature, i.e. dual-slope compensation. However, this prior approach requires calibration and trimming at two or more different temperatures, thereby increasing the cost of the overall sensor through the necessary calibration time for the circuit.
It is an object of the present invention to provide a non-linear temperature compensation circuit which is responsive to changes in operating temperature of the compensation circuit.
The present invention preferably provides a non-linear temperature compensation circuit for generating at least dual-slope characteristics responsive to changes in operating temperature of the compensation circuit. The inventive circuit can include a temperature dependent current generator circuit 2 for generating at least one output substantially proportional to changes in the temperature of the circuit from a first temperature. The non-linear temperature compensation circuit can also preferably include a current-based dual-slope drift generator for generating a current proportional to an absolute temperature. The novel circuit can further include summing means for summing the output generated by the temperature dependent current generator circuit and the current generated by the current-based dual-slope generator. In certain embodiments, this sum is used to generate the required voltage drift characteristic that can be applied to the sensor output.
The present invention is advantageous in that the cost to produce and calibrate is reduced due, in part, to the reduction in calibration time required. A further advantage of the inventive non-linear temperature compensation circuit is that the slope change points, or break points, can be known and do not require trimming for setting the slope change point.